NIST Technical Note 1637

Modeling the Effects of Outdoor Gasoline 

Powered Generator Use on Indoor Carbon 

Monoxide Exposures

Liangzhu (Leon) Wang

Steven J Emmerich
NIST Technical Note 1637

Modeling the Effects of Outdoor Gasoline

Powered Generator Use on Indoor Carbon 

Monoxide Exposures

Liangzhu (Leon) Wang

Steven J Emmerich

Building Environment Division

Building and Fire Research Laboratory

August 2009

U.S. Department of Commerce

Gary Locke, Secretary

National Institute of Standards and Technology

Patrick D. Gallagher, Deputy Director
iii

National Institute of Standards and Technology Technical Note 1637

Natl. Inst. Stand. Technol. Tech. Note 1637, 23 pages (July 2009)

CODEN: NSPUE2
1

Abstract

The U.S. Centers for Disease Control and Prevention (CDC) has reported that up to half 

of non-fatal CO poisoning incidents during the hurricane seasons in 2004 and 2005 

involved generators operated outdoors but within seven feet of the home. The guidance 

provided on the safe operating distance of a generator is often neither specific nor 

consistent. Furthermore, some generator manufacturers recommend the use of extension 

cords to be "as short as possible, preferably less than 15 feet long, to prevent voltage drop 

and possible overheating of wires." However, the use of short extension cords may result 

in placement of the generator too close to the home to reduce the likelihood of the entry 

of CO. This study modeled multiple scenarios of a portable generator operated outdoors 

using the CONTAM indoor air quality model coupled with a computational fluid 

dynamics (CFD) model to predict CO concentrations near and within a home. The 

simulation cases included both human-controllable factors (e.g., generator location and 

exhaust direction and window opening size) and non-controllable factors (e.g., wind, 

temperature, and house dimensions). For the house modeled in this study, a generator 

positioned 4.6 m (15 feet) away from open windows may not be far enough to limit CO 

entry into the house. It was also found that winds perpendicular to the open window 

resulted in more CO infiltration than winds at an angle, and lower wind speed generally 

led to more CO entry. To reduce CO entry, the generator should ideally be positioned 

outside of airflow recirculation region near the open windows.

Keywords

Generator; carbon monoxide; generator safe operating distance; CONTAM; 

computational fluid dynamics; indoor air quality; health; multizone airflow model; 

poisoning; simulation
2

Nomenclature

A Generator exhaust pointing away from the open window

Ag Wind with an angle

BL Larger of upwind building face dimensions

BS Smaller of upwind building face dimensions

D Downwind

DW Generator placed downwind to the open window

FR Family room

GD Generator placement distance from the open window

KIT Kitchen

LV Living room

LVFRKIT Living room, family room and kitchen

N The north direction

OS Size of the open window

P Wind perpendicular to the open window

PD Generator exhaust pointing direction

Perp Perpendicular wind

Rlw Size of leeward recirculation zone

S Simulation

T Temperature; Generator exhaust pointing Towards the open window

Tin Inside air temperature

Tout Outside air temperature

U Upwind

UW Generator placed upwind to the open window

WD Wind direction clockwise relative to the north

WS Wind speed

Greek Symbol

∆ Difference
3

Introduction

Gasoline-powered portable electric generators are widely used to provide heat and power

in U.S. households during power outages, especially during hurricane seasons. During 

Hurricane Isabel in 2003, portable generators were reported to be sold out in the 

Washington, DC metropolitan area (CPSC 2003). As a product of gasoline combustion, 

carbon monoxide (CO) from generator exhaust can be a significant safety and health 

issue. Users often place generators near or in their homes based on concerns about

generator theft and noise to neighbors (CPSC 2006). When a generator is operated

outside, the power cord often needs to go though a slightly open, unlocked door or 

window. An investigation of the U.S. Consumer Product Safety Commission showed that 

five out of 104 deaths caused by generator CO poisoning were associated with a 

generator that was placed outside the home near an open window, door, or vent (Marcy 

and Ascone 2005). The U.S. Centers for Disease Control and Prevention (CDC) has 

reported that 34 % of non-fatal CO poisoning incidents after hurricanes in Florida in 2004, 

and 50 % during Hurricanes Katrina and Rita in 2005 involved generators operated 

outdoors but within 2.1 m (7 ft) of the home (CDC 2006). However, the guidance for the 

safe operating distance of a generator is often neither specific nor consistent. Some 

guidance mentions that a generator should have “three to four feet of clear space on all 

sides and above it to ensure adequate ventilation” (OSHA 2005; FEMA 2006), whereas 

others required a generator not to be used “within 10 feet of windows, doors or other air 

intakes” (EPA 2005). While these guidelines seem to suggest keeping a generator at a 

certain distance from a house, some generator manufacturers recommend in their 

instruction manuals that power cords be “as short as possible, preferably less than 15 feet

long, to prevent voltage drop and possible overheating of wires” (CPSC 2006). The use 

of short extension cords may result in placement of the generator such that a significant 

amount of CO enters the home.

This paper presents a series of numerical simulations of the entry of CO from a generator 

exhaust into a one-story house. A matrix of simulation scenarios was created to consider 

multiple factors contributing to the CO entry, including human-controllable factors (e.g., 

the generator location and exhaust direction and window opening size) and noncontrollable factors (e.g., wind, temperature, and house dimensions). The transient 

indoor CO profiles were modeled using the CONTAM indoor air quality model (Walton 

and Dols 2005) integrated with a computational fluid dynamics (CFD) model, CFD0 

(Wang 2007), which was used to predict outdoor CO dispersion near the house. Several 

values of generator placement distance were evaluated under different weather conditions. 

The results of this study provide some important insights for operating a portable 

generator safely outdoors.
4

Problem and Method

Figure 1 shows a general schematic of airflow streamlines near a house and potential 

factors affecting house CO entry when a generator is placed upwind of a house. Once 

released from the generator exhaust, CO disperses in the vicinity of a house, and transfers 

through openings, i.e. open doors/windows/vents, with the air infiltrating into the house. 

The rate of CO entry into the house is related to the CO level nearby and the amount of 

air infiltration into the house. Multiple factors affecting outside CO dispersion include the 

generator placement distance (GD) from the house, the exhaust direction (PD), the 

generator being positioned either upwind (UW) or downwind (DW) of an opening, wind 

speed (WS) and direction (WD). In addition to the ambient wind condition affecting the 

entry rate, other factors include opening size (OS), and outdoor-indoor temperature 

difference (∆T = Tout – Tin). This study used as the opening a partially opened window as 

shown in Figure 1. 

Figure 1. Schematic of airflow streamlines and factors affecting house CO entry when a 

generator is placed upwind of a house.

These factors were grouped into two categories: human-controllable factors and 

environmental (non-controllable) factors. A matrix of simulations was developed to 

consider the combined effects of these factors as illustrated in Table 1. If all factors were 

considered together, a full combination would have included 432 (2 × 3 × 2 × 2 ×3 × 2 × 3) 

simulations. The strategy employed in this study was to focus on one or a group of 

factors to find the worst, medium, and/or best scenarios for house CO entry while 

keeping other factors as a fixed condition. With this method, the total number was 

reduced to 56 simulations.

Wind unaffected by house

Leeward 

recirculation zone

Tout

Tin

OS

GD

PD

UW DW

Windward recirculation zone

Generator

WS and WD
5

Table 1. Simulation parameter matrix

Focus of 

Study S

Human-controllable Factors Environmental Factors

PD GD(m) UW/

DW OS(m2

) WS(m/s) WD 

( ̊) ∆T ( ̊C)

T A 0.9 4.6 7.6 U D 0.05 0.31 1 5 10 P Ag 0 10 -20

Gen. 

Pointing 

Direction

1 X X X X X X X

2 X X X X X X X

3 X X X X X X X

4 X X X X X X X

5 X X X X X X X

6 X X X X X X X

7 X X X X X X X

8 X X X X X X X

Gen. 

Distance 

& 

Window 

Size

1 X X X X X X X

2 X X X X X X X

3 X X X X X X X

4 X X X X X X X

5 X X X X X X X

6 X X X X X X X

7 X X X X X X X

8 X X X X X X X

9 X X X X X X X

10 X X X X X X X

11 X X X X X X X

12 X X X X X X X

Envtl. 

Factors

1 X X X X X X X

2 X X X X X X X

3 X X X X X X X

4 X X X X X X X

5 X X X X X X X

6 X X X X X X X

7 X X X X X X X

8 X X X X X X X

9 X X X X X X X

10 X X X X X X X

11 X X X X X X X

12 X X X x X X X

13 X X X X X X X

14 X X X X X X X

15 X X X X X X X

16 X X X X X X X

17 X X X X X X X

18 X X X X X X X

19 X X X X X X X

20 X X X X X X X

21 X X X X X X X

22 X X X X X X X

23 X X X X X X X

24 X X X X X X X

25 X X X X X X X

26 X X X X X X X

27 X X X X X X X

28 X X X X X X X

29 X X X X X X X
6

Focus of 

Study S

Human-controllable Factors Environmental Factors

PD GD(m) UW/

DW OS(m2

) WS(m/s) WD 

( ̊) ∆T ( ̊C)

T A 0.9 4.6 7.6 U D 0.05 0.31 1 5 10 P Ag 0 10 -20

30 X X X X X X X

31 X X X X X X X

32 X X X X X X X

33 X X X X X X X

34 X X X X X X X

35 X X X X X X X

36 X X X X X X X

S: simulation; PD: pointing direction of generator exhaust; GD: generator distance from the open window; 

UW/DW: generator upwind/downwind to the open window; OS: open window size; WS: wind speed; WD: 

wind direction relative to the north; ∆T: outdoor and indoor temperature difference; T: generator exhaust 

pointing towards the open window; A: generator exhaust pointing away from the open window; U: upwind; 

D: downwind; P: wind perpendicular to the open window; Ag: wind with an angle

Specifically, the study was ordered as follows. The human-controllable factors were 

studied first with the weather condition fixed at a 5 m/s wind perpendicularly blowing 

towards the open window and zero outdoor-indoor temperature difference. The first of 

the human-controllable factors investigated was the pointing direction of generator 

exhaust, which was either towards (T) or away from (A) the open window, to find out the 

exhaust pointing direction that allowed more CO entry for use in the later simulations. 

Other settings include an open window size of 0.31 m2(a 12 in opening for a window 

width of 39.4 in), the generator placed either upwind or downwind with a distance of 

0.9 m (≈3 ft as comparable to the literature) or 4.6 m (15 ft) away from the window. The 

combination of multiple factors ended up with eight simulations as shown in Table 1. 

The second focus of the study was the human-controllable factors: placement distance of 

the generator and size of the open window. A generator distance of 0.9 m, 4.6 m, or 7.6 m 

(3 ft, 15 ft, 25 ft) and an open window size of 0.05 m

2

(a 2 in crack with the window 

width of 39.4 in equivalently) or 0.31 m2 were studied. This set of variables was used to 

find the worst, medium, and best scenarios of CO entry in the house considering the 

generator placement distance and open window size. Using the three scenarios as 

baseline cases, the study then investigated the impacts of environmental factors by 

changing the ambient conditions. The variable environmental factors included wind 

speeds of 1 m/s and 10 m/s; wind directions either perpendicular to the open window or 

at an angle; outdoor-indoor temperature difference of 0 oC, 10 oC, and -20 oC. These 

factors result in a total of 36 simulations.

One important parameter that was not varied during the simulation study was the 

emission rate of CO from the generator. This was kept at 1 kg/h for all cases. Based on 

tests conducted at NIST, this is a reasonable estimate for a 5 kW generator operated near 

full load. In reality, the CO emission from any generator will depend on a variety of 

factor including the engine size and type, the connected electrical load and ambient 

conditions.

The house modeled in this study was based on a manufactured house on the campus of 

the National Institute of Standards and Technology (NIST). An aerial view and inside 

view of the house are shown in Figure 2(a) and Figure 2(c), respectively. The house 
7

includes three bedrooms, living room (LV), family room (FR), kitchen (KIT), and an 

attached garage. 

 Figure 2(a). Aerial view of the house. Figure 2(b) Meshed house in CFD.

Figure 2(c). Inside view of the house.

Figure 2(d). The house modeled in CONTAM.

Figure 2. The manufactured house to be modeled.

Generator

N

N

Garage

Family 

Room 

(FR)

Kitchen 

(KIT)

Living 

Room 

(LV)

Master Bedroom

Bed 

Room3

Bed 

Room2

Open window 

N

Garage

Family 

Room 

(FR) Kitchen 

(KIT)

Living 

Room 

(LV)

Master 

Bedroom

Bed 

Room2

Bed 

Room3

Open window 

N
8

A simulation to predict external CO dispersion and internal CO transient profiles 

involved a two-step procedure. First, a computational fluid dynamics (CFD) program, 

CFD0 (Wang 2007), was used to simulate the external airflow and CO dispersion around 

the house. Figure 2(b) shows the orthogonal structured mesh used in one of the CFD 

simulations when a generator was placed a certain distance from the house. The CFD 

simulations predicted CO distribution at the house envelope corresponding to different 

wind directions, which was saved in a database file. The file contained CO concentrations 

for each point on the house envelope for each wind direction simulated. In considering 

the surrounding wind condition correctly, a profile of atmospheric wind for “open 

terrain” (ASHRAE 2005) was used based on the surrounding condition of the actual 

house. Figure 3 illustrates the wind profiles used in the simulations for a reference wind 

speed of 1 m/s, 5 m/s, and 10 m/s at the height of 10 m (33 ft) above the ground.

Figure 3. Wind profiles for the reference wind speeds of 1 m/s, 5 m/s, and 10 m/s at the 

height of 10 m above the ground.

The second step in the effort was to predict transient CO profiles in the house using 

CONTAM. Figure 2(d) shows the house as modeled in CONTAM. The CONTAM model 

uses the CFD results for CO concentration for each opening/crack of the house as the 

entering concentration for any air infiltrating at that crack/opening. The model is then 

used to predict a CO profile for each room of the house for a period of eight hours, which 

is a reasonable runtime for a generator (Brown 2006). 

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6 7 8 9 10

Wind speed(m/s) Height(m) Ref wind speed(1m/s) Ref wind speed(5m/s) Ref wind speed(10m/s)
9

The CO hazard was evaluated using both the peak value of CO level in the house and the 

resulting percentage of carboxyhemoglobin (%COHb) level. %COHb is one of the most 

commonly used indicators for CO exposure (Haloulakou et al. 2000; Inkster 2004) and 

can be predicted from mathematical models. One of the most precise mathematical 

models is the Coburn-Foster-Kane (CFK) model, which has been broadly used and 

validated by many previous studies (Peterson and Stewart 1975; EPA 2000; EPA 2001). 

The CFK model takes into account important variables such as exposure duration, 

alveolar ventilation, partial pressure of CO in the inhaled air, blood volume, diffusivity of 

the lung for CO, and rate of endogenous CO production (Stewart 1975). Equations (1) 

and (2) show the CFK equation in its integral form during a time period of ∆t. The peak 

value of the %COHb in the house provides a criterion of CO exposure for different 

simulations under various affecting factors.

) t

BV

P

MBV ([OHb ] [COHb ] )

[COHb ] P

V

V

[COHb ] [COHb ] (

b

CO

b max t 1

t 1 O

b

CO

t t 1

2  



  







 ( 1 )

(EPA 2001)

100

[OHb ]

[COHb ]

[%COHb ]

max

t

t

  ( 2 )

where PCO, partial pressure of CO in the air inhaled (mm Hg), was determined by

P C 1316 CO

CO

, in which CCO is the CO concentration in ppm(v). The initial [COHb]0

is typically 

3

1.659 10

ml/ml for a non-smoker, which is equivalent to the [%COHb]0 of 

0.75 % (EPA 2001).
10

Results and Discussions

The fifty-six simulations in Table 1 were conducted, which focused on the effects of 

generator exhaust direction, generator placement distance from the house, size of the 

open window, and environmental factors.

Human-controllable factor: generator exhaust pointing direction

Generator users may point the exhaust towards an open window as shown by Figure 4(a) 

or away from the window in Figure 4(b). As defined in Table 1, eight simulations were 

conducted for the generator exhaust pointing direction when the generator was placed 

0.9 m (3 ft) or 4.6 m (15 ft) upwind or downwind of the open window. A wind 

perpendicular to the south wall of the house was set as 5 m/s fixed and the outdoor-indoor 

temperature difference was set to 0 oC.

Figure 4. Study of generator exhaust pointing direction.

Figure 5 presents the eight simulations of generator exhaust direction for the different 

rooms in the house. Note that the living room (LV), family room (FR), and kitchen (KIT) 

were modeled and reported as a single zone of LVFRKIT in Figure 5 and afterwards in 

this paper. As shown in Figure 5(a), when the generator was placed 0.9 m upwind from 

the house, the maximum CO level occurred in the LVFRKIT for both cases with the 

exhaust pointing towards (S1) and away from (S2) the open window. It seems that the 

exhaust pointing direction does not significantly affect CO entry, with a maximum 

concentration of 142 mg/m3in case S1 and 134 mg/m3in case S2. In the cases S3 and S4, 

for which the only difference was a wind direction of North and the generator downwind 

(DW) of the open window, the maximum CO concentration occurred in Bedroom 3, 

although the CO level itself was small. The wind direction thus seemed to play a more 

important role than the exhaust pointing direction. 

Generator exhaust

Wind

Generator exhaust

Wind

N N

Figure 4(a). Generator exhaust 

pointing towards the house.

Figure 4(b). Generator exhaust 

pointing away from the house.
11

Figure 5(a). Maximum CO levels in the house for the generator exhaust pointing 

directions when the generator was placed 0.9 m from the house.

Figure 5(b). Maximum CO levels in the house for the generator exhaust pointing 

directions when the generator was placed 4.6 m from the house. 

Figure 5. Results for generator exhaust pointing directions.

S1(LVFRKIT)

S2(LVFRKIT)

0

20

40

60

80

100

120

140

160

0:00:00 1:00:00 2:00:00 3:00:00 4:00:00 5:00:00 6:00:00 7:00:00 8:00:00

Time(h) CO(mg/m

3

)

S1(LVFRKIT) S2(LVFRKIT) S3(Bed Room3) S4(Bed Room3)

S3&S4(Bed Room3)

S5(LVFRKIT)

S6(LVFRKIT)

0

5

10

15

20

25

0:00:00 1:00:00 2:00:00 3:00:00 4:00:00 5:00:00 6:00:00 7:00:00 8:00:00

Time(h) CO(mg/m3)

S5(LVFRKIT) S6(LVFRKIT) S7(Bed Room3) S8(Bed Room3)

S7&S8(Bed Room3)
12

Unsurprisingly, the generator placement distance was also a significant factor to the 

house CO entry. Figure 5(b) illustrates that when the generator was placed 4.6 m (15 ft) 

from the house, the maximum CO level was significantly less than the corresponding 

case when the distance was 0.9 m (3 ft). For simulation S5, the CO level was only 

21 mg/m3in LVFRKIT compared to the 142 mg/m3in S1. Similar trends were observed 

for S6 through S8. Overall, these simulations show that compared to the exhaust pointing 

direction, the generator placement distance and the wind direction were more important 

factors, which were analyzed further. The following simulations employed an exhaust 

pointing direction t towards the open window, since it generally causes higher CO entry 

than the exhaust pointed away from the house.

Human-controllable factors: generator placement distance and open window size

The distance of generator placement from the house is a user’s decision. Apparently, 

increasing the distance aids in avoiding CO hazards. Other concerns, however, may 

impact the generator location, including concerns about the generator being stolen, noise 

to neighbors, and limited extension cord length (CPSC 2006). Therefore, a minimum 

distance for safe generator operation outdoors is of great interest. However, the answer is 

not straightforward, as CO dispersion around a house is significantly affected by the 

aerodynamics near a house, which is closely related to house geometry, e.g. shape and 

dimension, and weather conditions such as wind speed and direction. 

A simple demonstration can be made by revisiting Figure 1. The interaction of the house 

and the wind creates two major eddies, i.e., windward and leeward recirculation zones, 

with the latter generally being wider than the former. The existence of these two 

recirculation zones may “trap” CO to create a local region with fairly high CO levels. 

Figure 6 shows more clearly how the recirculation zones affected CO dispersion around 

the house. When the generator sits upwind 0.9 m (3 ft) away from the house but inside 

the windward recirculation zone, i.e. Figure 6(a), much of the CO is trapped at the corner 

of the house, which potentially could cause more CO entry into the house. If the 

generator is moved farther away from the house, e.g. 4.6 m in Figure 6(c) and 7.6 m in 

Figure 6(e), the CO is diluted by the wind to achieve a lower level near the house. A 

similar observation could be made when the generator is placed downwind of the house 

in Figures 6(b), 6(d), and 6(f), with the exception that the leeward recirculation zone is 

generally larger than the windward one. As a result, a distance of 4.6 m (15 ft) may not

be far enough as shown in Figure 6(d). When the distance is increased to 7.6 m (25 ft), 

CO is well diluted outside the recirculation zone (Figure 6(f)). 

Although this study modeled a specific case, it shows that a minimum distance of safe 

generator operation could be closely related to the recirculation zones of the house, 

especially the leeward zone, which is often larger than the windward one. An empirical 

estimate of the size of the leeward recirculation zone (Rlw) for a general case is provided 

by Equation (3) (ASHRAE 2005): 

Rlw = BS

0.67BL0.33

 (3)
13

where BS is the smaller of upwind building face dimensions; BL is the larger of upwind 

building face dimensions of building height and width. When BL > 8BS, BL = 8BS should 

be used. When a building has varying roof levels or wings separated by at least a distance 

BS, the building dimensions should be determined only by the height and width of the 

building face below the portion of the roof in question (ASHRAE 2005).

Applying Equation (3) to the house in this study, Rlw = 6.7 m (22 feet). Therefore a 

generator distance of 7.6 m (25 feet) from the house is far enough to help avoid CO being 

trapped in the leeward recirculation zone. 

 

Figure 6(a) Figure 6(b)

 

Figure 6(c) Figure 6(d)

 

Figure 6(e) Figure 6(f)

Figure 6. The study of generator placement distance and open window size. Streamlines 

and CO levels for the simulation (a) S1 and S2; (b) S3 and S4; (c) S5 and S6; (d) S7 and 

S8; (e) S9 and S10; (f) S11 and S12.

After predicting the CO levels near the house, the CO entry rate into the house can be 

calculated considering the effect of the open window size. Figure 7 compares the peak 

CO and %COHb levels in the house for different generator distances and open window 

sizes. The worst scenario of CO entry, with the highest CO level of 142 mg/m3and 

%COHb = 16.4 %, occurred in simulation S1. In S1, the generator was placed 0.9 m 

upwind to the house with an open window size of 0.31 m2. As a comparison, the best case 

was the simulation S12, where the generator distance was 7.6 m downwind and the open 

window size was 0.05 m2. The simulation S5 was found to be the medium case, with the 

CO value closest to the average value of all 12 cases, 27 mg/m3(3 %COHb). The worst, 

N N

N N

N N
14

medium, and best scenarios were used as baseline cases to populate a series of 

simulations with the focus of studying non-controllable environmental factors on CO 

entry. 

Figure 7. Maximum CO levels and %COHb for generator placement distance and open 

window size.

Note from Figure 7 that the predicted peak CO levels in the house were not as high as 

expected when the generator was placed downwind as in S3, S4, S7, and S8, although the 

CO was trapped inside the leeward recirculation zones in Figures 6(b) and 6(d). This 

difference occurred because, for the specific case modeled in this study, the predicted 

airflow direction of the open window was all outflow (from the house to the ambient) and 

CO was not brought into the house as a result. Other situations could occur that would 

reverse the airflow direction and bring CO into the house. Therefore, due to the existence 

of leeward recirculation zones with high CO levels, S3, S4, S7, and S8 were still 

considered CO hazardous cases. A generator distance of 4.6 m (15 ft) was considered not 

safe enough to avoid excessive CO exposure. It was also noted from Figure 7 that both 

levels of %COHb and maximum CO are useful evaluation criteria of CO hazards. To 

simplify the analysis, maximum CO levels are used as the only CO hazard evaluation 

criterion in the remainder of this paper.

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

0

20

40

60

80

100

120

140

160

1 2 5 6 9 10 7 3 11 8 4 12 Max %COHb Max CO(mg/m3

)

Simulation

Max CO Max %COHb

Worst (S1)

Best(S12)

Medium(S5)

Upwind Downwind

Averge Value
15

Non-controllable factors: environmental factors

The studies of human-controllable factors assumed a single set of weather conditions: 

5 m/s wind speed with wind perpendicular to the south/north wall, and an outdoor-indoor 

temperature difference of 0 °C. With the worst, medium, and best cases identified under 

these weather conditions, additional simulations were performed to investigate how 

weather conditions affect CO entry. As shown in Table 1, a total of 36 simulations were 

conducted with wind speed of 1 m/s or 10 m/s, a range of wind directions, and 

temperature differences of 0 °C, 10 °C, and 20 °C. In fact, the actual total number of 

simulations was over 36 because a series of simulations was tried for a range of wind 

angles from 0° to 330° clockwise from the north. It was found out that when a wind was 

210° clockwise from the north, the CO level at the house envelope was generally higher 

than other wind angles. Therefore, this wind direction was selected as a representative 

case for wind at an angle.

Figure 8. Maximum CO levels showing the effects of environmental factors on CO entry.

Figure 8 compares the maximum CO levels in the house for various weather conditions. 

Generally, a lower wind speed is associated with a higher rate of CO entry. This effect 

can be explained by a stronger wind facilitating CO dilution and causing the CO in the 

recirculation zones to disperse more easily. A higher wind speed also causes more air 

infiltration into the house so that the CO level in the house was diluted more quickly. In 

addition to the wind speed effect, the generator placement distance plays a major role in

CO entry. The CO concentration reaches 715 mg/m3(%COHb = 51 %) when the 
16

generator was 0.9 m upwind of the open window for a wind speed of 1.0 m/s in 

simulation S3. Wind direction was another major factor. A sharp drop of CO levels could 

be observed when the wind was from an angle rather than perpendicular to the open 

window. However, when the generator was moved far enough away from the house, i.e. 

7.6 m (25 ft), the peak CO levels were minimal regardless of wind speed and direction. A 

distance of 7.6 m (25 ft) thus seemed to be reasonably safe for the house in question. 

Variation in CO levels occurred for the generator distance of 4.6 m (15 ft), which could 

be as high as 89 mg/m3(%COHb = 11 %), showing that 4.6 m (15 ft) was not necessarily 

an acceptable operating distance for the house modeled in this study. 

The outdoor-indoor temperature difference, ∆T, affected the CO entry indirectly by 

contributing to the infiltration of outdoor air. The air infiltration pattern was not 

determined only by the wind effect but rather by a combination of buoyancy and wind 

effects, which can be shown by comparing S1 and S2 in Figure 8. When ∆T was zero, as 

in S1, and wind speed was just 1.0 m/s, the air flowed into the open window and brought 

CO into the house. Whereas under the same conditions, except with an outside 

temperature 10 °C higher than the inside in S2, the buoyancy effect dominated the wind 

effect and the airflow direction was reversed. The green bar in Figure 9(a) shows the 

airflow at the open window became an outflow in S2. Consequently, the peak CO in the 

house for S2 dropped sharply as shown in Figure 8. When the wind speed was strong 

enough, e.g. 10 m/s in S19 – S21, the buoyancy effect caused by the change of ∆T was 

insignificant compared to the wind effect. The airflow at the open window was thus 

always inflow (i.e., the green bar for S20 in Figure 9(b)). The variation of CO levels of 

S19-S21 in Figure 8 was minimal for a range of ∆T from -20 °C to 10 °C. 
17

Figure 9(a)

Figure 9(b)

Figure 9. CONTAM simulation results of airflow rates and pressure drops for the 

simulation (a) S2 and (b) S20 for the study of environmental factors.

Open window

Open window 
18

Conclusion

Through a series of simulations on a one-story manufactured house, this study found that 

positioning a generator 4.6 m (15 feet) from open windows may not be far enough for the 

house modeled to avoid excessive CO entry. Ideally, the generator should be positioned 

outside of airflow recirculation regions near the open window. As for human noncontrollable factors, a perpendicular wind to the open window often led to more house 

CO entry than wind with an angle. Lower wind speed generally caused more entry of CO 

when the outdoor-indoor temperature difference was relatively small so that the CO entry 

by buoyancy effect could be neglected. When the buoyancy effect was significant, the 

infiltration of airflow and CO were determined by the combined forces of wind and 

buoyancy. Major CO entry into the house occurred primarily when the generator was 

placed inside the airflow recirculation zone, the size of which was found to be related to 

the dimensions of the house. General guidance for the safe operating distance of a 

generator could be developed considering the size of the airflow recirculation zone and 

the house dimensions.
19

Acknowledgements

This work was sponsored by the US. Centers for Disease Control and Prevention under 

Interagency Agreement No. 08FED894639.

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